Imaging system employing ions

ABSTRACT

In the formation of visible copies of an image, the use of an ion modulating array provided with an asymmetrical photosensitive coating together with means to electrostatically develop the ion image.

United States Patent 11 1 3,797,926 Fotland et a1. 1 Mar. 19, i974 1 1IMAGING SYSTEM EMPLOYING IONS 3.603.790 9/1971 Cleare ass/17 x 5]Inventors: Richard A. Foland a e me 3.582.206 6/1971 Burdige 355/16Heights; Virgil E. Straughan, FOREIGN PATENTS OR APPLICATIONS Eudid bothof Ohio 1.156 308 10/1963 Germany 1. 96/1 R Assigneei HorizonsIncorporated, a Division 1.152.308 V 5/1969 Great Britain 355/3 HorizonsResearch Incorporated, OTHER PUBLICATIONS Cleveland Ohm DefensivePublication, T879,010, L. F. Frank, 0m. [22] Filed: Aug. 27, 1971 1970,Exposure Latitude in Electrophotographic Sys- 211 App]. No.: 178,521

Primary Examiner-Robert P. Greiner [52] US. Cl 355/3, 355/16, 355/17, At r y Agent; or Firm--Lawrence I, Field 96/1 R, l17/17.5 [51] Int. CL...G03g 15/00 [58] Field of Search 355/3, 16, 17; 96/1 R; [57] ABSTRACT117/ 7 5 In the formation of visible copies of an image, the use of anionmodulating array provided with an asymmet- 56 R fer n Ci ricalphotosensitive coating together with means to UNITED STATES PATENTSelectrostatically develop the ion image. 3.645.614 2/1972 McFarlane etal 355/3 I 21 Claims, 17 Drawing Figures dimer PAIENIEDMAMSIQM 3.797.926

sum 1 or 2 FIG. I. 1 22 I 24 LJ INVENTORS Richard A. Foflond Virgil E.Sfraughon BY ida m yaw ATTORNEY Pmmmmwwn 3791.926

INVENTORS RichdrdA1FofIond Virgil E. Sfraughon I y w f u 75414 4ATTORNEY This invention relates to image reproduction and ficientformation of ion current patterns corresponding to an optical image. 7

In conventional plain paper electrostatic photography, an insulatingphotoconductor is charged with a corona source of ions, exposed, thecharge image developed, the developed image transferred to plain paper,and finally, the toned image is fixed, generally by fusing. After thetransfer operation, the residual image is erased from the surface of thephotoconductor and the photoconductor is cleaned in preparation of arepetition of the process. Although employing plain paper, this processis complicated by the requirement for a number of different machineoperations. In addition, the photoconductor suffers wear over a periodof time, since thesurface or the photoconductor is repeatedly rubbed bytoner particles, cleaning brushes and paper surfaces.

A related process employs a photoconductively coated conducting paper.The photoconductor, generally zinc oxide (although organicphotoconductors may be employed), is first charged, then exposed, andthe image toned. Here'the photoconductor is not resuable and thus thewear and tear restrictions in the aforementioned process are eliminated.In addition, the machine operation, requiring four steps, is simplified;The disadvantage of this process is associated with the requirementfor'coating the paper with a photoconductor. These photoconductivelycoated papers are significantly more expensive than plain uncoatedpaper. In addition, because of the heavy photoconductor coating (thecoating weight generally amounting to pound s per 3,000 ft ream),the'papers are heavy and have a feel quite different'fro'm plain paper.

A principal object of the present invention is to simpiify theconventional plain paper electrophotographic process and theapparatusby'which it is carried out.

.Another object ofthe invention -is to provide an image reproductionmethod wherein there is no physimore particularly to a method andapparatus for the efcal contact of the photoconductor with either devel-I oper or paper. Y 4

In addition to having the advantages of eliminating photoconductor wearand simplifying the number of machine operations, the method andapparatus of this invention do not require a photoconductively coatedpaper. In comparison therefore to electrostatic copy processes employingconductive paper, the process of this invention has the advantage oflower paper cost and the advantage of a capability for employing "plain"(non-chargeable) paper orv a dielectric coated paper which has the feel,weight and appearance of a plain bond paper. v} m Another-object of theinvention is to provide an image copying meansv whereby photoconductordefects, attracted dust and the like do not appear in the final copyithese defects being integrated out during an exposure. i 1 I a In the'present'invention, a fine mesh screen or grid coated with aphotoconductor is employed to spatially modulate the. flow of coronacurrent in accordance with anoptical image projected onto said fine meshscreen or grid. A I

which is coated with alight sensitive material, is det The use of a finewire meshor screen, the surface of scribed in US. Pat. No. 2,676,100 andin U.S. Pat. No.

In some respects thepresent invention is similar to that described inUS. Pat. No. 3,220,324, which discloses an apparatus and method offorming an electroconductor is utilized. As a consequence of thischange,

it is possible to obtain a contrast ratio of 6 to 11. One preferredembodiment of the present invention employs an insulating screen-wovenfrom a Nylon or'Dacron monofilament which is coated on one side with anelectrically conductive material and coated on the other side, in anasymmetrical fashion, with a photoconductor. By utilizing such a screen,contrast ratios as high as 10 or 12 are obtained. 7

The method of formingthe screen and the apparatus for utilizing thescreen for the formation of. visible images will be more fully apparentfrom the description which follows taken with the drawings in which:

FIG. 1 is a schematic view of an apparatus for preparing electrostaticchargelimages corresponding to a projected optical image upon an imagereceptive surface;

FIG. 2 is a cross section view of a modulating conductive screenillustrating the asymmetric nature of the photoconductive coatingthereon; i

FIG. 3 is a similar cross section illustrating the geometry of adielectric or insulating screen coated asymmetrically with both aconducting layer and a photoconductor;

FIG. 4 is a fragmentary view of a section through a metal plate having aplurality of apertures and which is asymmetrically coated with aphotoconductor;

FIGS. 2A, 3A .and 4A are enlarged views, in section, of portions ofFIGS. 2, 3 and 4;

FIG. 5 illustrates schematically a means for moving thephotoconductively coated screen and the corona wires during an exposure;

FIG. 6 illustrates a means for continuously supplying a fresh modulatingscreen during the operation of a copy device;

FIG. 7 is a schematic illustrating a means for generating full colorcopies of an original in a manner which eliminates registrationproblems;

FIGS. 8, 9 and 10 illustrate various modifications of apparatusincluding photoconductively coated screens in copy operations whereinthe final image is formed upon plain paper, that is, paper which is notcapable of sustaining a charge image. In FIG. 8, a toned image is fonnedupon a plastic film, which is in the form of an endless belt, and issubsequently transferred to a plain paper. FIG. 9 employs a simultaneouscharging and dev'eloping operation using a liquid or dry aerosol, whileFIG. 10 employs liquid development, the charging and development onceagain being carried. out simultaneously; v FIG. 11 illustrates anembodiment of the invention which employs a screen grid to electricallyisolate a 3 chargeable members surface potential from the screenpotential; and

FIGS. l2, l3 and 14 schematically depict devices em- 7 ployingv meansfor simultaneously exposing, charging,

and developing a visible image upon plain" paper, i.e., non-chargeablemembers, and utilizing the modulating screen of the present invention.

Referring now to FIG. 1, illustrating an apparatus for preparingelectrostatic images on an image receptor surface, the apparatuscomprises an electrically conductive platen 10 upon which is supported aconducting paper 12, having a thin dielectric coating 14. A coronamodulating screen, grid or aperture plate 16 controls the ion currentreaching the surface of the dielectric paper in accordance with opticalimage projected onto element 16. A corona source is provided, which maycomprise a fine wire 18. The corona operating potential is supplied bypower supply 20. The paper support substrate 10 is maintained at aselected potential provided by power supply 21. Electronic controls 24provide a means for simultaneously turning on power supplies 20, 21 and24 and an illumination source for a projector 22. Projector 22 providesthe image which is to be copied; this image being focused upon screen16.

Although in this embodiment the optical image is provided by a projectorsuch as might be employed in the projection of microfilm images toobtain hard copy, it will be understood that projector 22 could bereplaced by a cathode-ray tube display using a projection lens system orby an original document support plus a projection system forconventional ofi' ce copy, or any other suitable source of optical imagedepending upon the application of the apparatus. v

A single corona wire 18 is shown iri FIG. 1. In order to provide auniform corona over a large area, a plurality of corona wires may beutilized all connected in parallel to' power supply In order to providesufficient corona current, the corona wire diameter should be less than10 mils and to simplify handling of the wire, the wire diameter shouldbe greater than l mil. A preferable wire diameter for this applicationis 3 mils. Using a single corona wire spaced approximately 1 inch abovemodulating screen 16, uniform charging, in accordancewith the projectedoptical image, of the dielectric paper occurs over an area equal to thelength of the corona wire and a distance between l and 2 inches normalto the direction of the corona wire at the paper. In order to providefor more uniform charging, the corona wire(s) may be moved, in a planeparallel to the screen, during the exposure.

A dielectric paper is shown in FIG. 1, such papers being available fromavariety of paper mills and being employed widely in high speed computerprinters and recorders. The dielectric coated paper may be replaced withany of a wide variety of plastic films ranging in thickness from 0.1 to5 mils. Images have been successfully formed on both polyester andacetate films; and, indeed, any film which has a dielectric relaxationtime in excess of a few seconds and which falls within theaforementioned thickness range may be employed in the apparatus of FIG.1.

Means for mechanically transporting the dielectric paper or plastic filmunder the corona modulating screen, maintaining said paper (film)stationary during the exposure, and then removing the paper from theimaging station are not shown in FIG. 1; these mechanica] features beingwell known to those skilled in the art.

FIG. It shows the corona modulating screen maintained at groundpotential. In this event, the potential on the corona wire and backingplate 10 must be opposite in polarity. Thus, if the corona wire ismaintained at a positive potential, the backing plate must be maintainedat a negative potential so that positive ions emitted from the coronawire are accelerated to the dielectric paper after passing through themeshes of screen i6. Alternately, the backing plate 10 might bemaintained at ground potential, screen 16 at a positive potential, andcorona wire 18 at an even higher positive potential.

The potential required between corona wire 18 and screen 16 must be atleast sufficient to initiate a corona current, i.e., at least 4 to 5 kv.The higher the potential the greater the ion current and hence the morerapidly dielectric paper may be charged and the lower the requiredexposure time. The upper limit of corona potential is realized whensparking occurs between corona wire 18 and screen 16. This is, ofcourse, a function of the spacing between 16 and 18. Corona potentialsas high as 25 kv have been employed in this invention successfully.

The potential required between screen 16 and backing plate 10 dependsupon the spacing between said members and the required resolution of theelectrostatic image formed on the charge supporting member. if thepotential for a given spacing is too high, sparking will occur betweenthe chargeable member and screen 16. Furthermore, at high potentials fora given spacing, the resolution of the charge image is sufficiently highso that a screen pattern corresponding to the screen 16 is observed inthe charge pattern laid down on the chargeable member. A preferredelectric field, in this region, is 20 kv per inch. This corresponds toan ap' plied potential dflO kv at a 6 inch spacing or i lav a 50 milspacing. At this electric field the corona CL rent passing throughscreen 16 and onto the chargeabiq member follows the field linesufficiently well so that a resolution of 6 to 10 line-pairs/mm isreadily obtained with screens havingfrom 240 to 325 meshes per inch. Atelectric fields in the range of 50 to I00 kv per inch, sparkingoccasionally occurs and the screen mesh pattern appears in the image. Atfields below approximately 3 kv per inch, ion spreading is observed withsubsequent degradation of image resolution.

The exposure times required are a complicated function of the coronavoltage, corona-to-screen spacing, light intensity at the screen, natureof the photccoriductor, and also the nature of the charge receivingmember and the type of development employed in converting theelectrostatic image into a visible image. In general, the requiredscreen illumination ranges from I to 50 ft.-candles of tungstenillumination and the exposure times range from 0.1 to 3 seconds.

FIG. 2 is a cross-sectional view of a wire mesh screen coatedwith aphotoconductor. The wire mesh 30 may be formed of any available metal oralloy, typical materials including brass, stainless steel, aluminum orphosphor bronze. The mesh size, i.e., the nurnlzers of wires per linearinch, may range from to 1,009. A 100 mesh screen will provide aresolution of 2 to 4 linepairs/rnm while a 325 mesh screen is capable ofproviding 7 to 14 line-pairs/mm. The photoconductive coating 32 is shownhere as being offset by an angle of 45 from the normal. By forming thephotoconductor in an asymmetrical manner such as this, much highercontrast ratios, i.e., ion current transmissivity ratio be- Onepreferred way of applying the photoconductor I to the screen 30 is byvacuum vapor deposition. The materialto be vaporized is placed in acrucible or metal container which is electrically resistance heated. The

screen to be coated is supported above the crucible at an anglegenerally 45 with the normal. The angular orientation of this screen,while mounted 45, is not critical. Thus, either the warp or woof of theweave may be parallel to the ground or run at any angle to the groundwithout adversely affecting theincrease in contrast ratio. In addition,during the evaporation, the screen may be rotated .through some angle inorder to form more complex asymmetrical patterns.

In FIG. 2A, a single elementof the array is shown, in section, showingthe manner in whichthe photosensitive coating 32 is asymetricallydisposed on the base 30. FIG. 3A isa similar view showing thedisposition of both the photoconductive coating 37 and the electricallyconductive coating36 on the insulating filament base 34. FIG. 4A is asimilar view of the array in FIG. 4.

FIG. 3 shows a. preferred embodiment of the present invention; thescreen 34 being fabricated from an insulating material. Typicalinsulating materials employed in this invention are woven fabricsconsisting of monofilament nylon, polypropylene, polyester or polyamide.Such woven fabric screens'are available in mesh sizes to' over 325 mesh,are extremely strong andare much less expensive than corresponding metalwoven screens. Furthermore, by employing an insulating screen having aphotoconductor on one side and a continuous conducting layer on theother side, higher contrast ratios are obtained than with plain metalscreens. In FIG. 3, the dielectric mesh 34 is shown having aconductivecoating 36 on the bottom anda photoconductive coating 37 onthe top and offset from the normal by 45. Instead of offsetting thephotoconductor, as

shown, thescreen may be formed by evaporating the photoconductor 37normal to the surface, with a consequent reduction in contrast ratio.The conductive layer is preferably formed by vacuum vapor deposition ofan FIG. 4, which is a cross sectional view of an aperture plate. Thesupporting plate'38, having a thickness in the range of l to Sriiils,may be fabricated from etching a plurality of holes through the surfaceand then coating the material with a photoconductor at an angle from thenormal as shown in FIG. 4.'Alternately, the plate 38 may befabricatedfrom a plastic sheet which also contains a plurality of holes etched inthe surface. In the case of an insulating support sheet, a conductorwould be deposited on the sides of the holes and bottom of the plate inamanner similar to that shown in FIG. 3.

The mesh screens may be woven with either a plain square weave or atwill-square weave. With a twill square weave, however, the resolutionin one direction is degraded slightly.

' In all cases, a higher contrast ratio is observed if the screen ismounted so that the photoconductor coated side faces the corona wires. Asomewhat lower contrast is obtained with the photoconductor coated sidefacing away from the corona wires.

One of the many advantages of the present invention in comparison toconventional electrostatic photographic systems involves a relaxation inthe requirements for high dark resistivity of the photoconductor. Atypical selenium xerographic plate or drum has a capacity close to 100pf/cm. If such a plate is charged to 500 volts and the allowable voltagedecay must be 100 volts or less in a period of 1 second (the minimumtime interval between charging and image development), thena simplecalculation will show that the dark current through the plate must beless than 10 amp/cm or the plate dark resistance must be in excess of 5X 10' ohm/cm of plate area. In a typical screen modulation apparatus, asdescribed herein, the corona current to the screen might be in the rangeof 3 l0 amp/cm". In order to provide effective modulation of the screencorona current, it is estimated that a voltage drop of at least I00volts is required across the photoconductor coating of thescreen. Thus,the photoconductor resistance in the dark must be in excess of 3' X 10ohm/cm of the screen area. This represents a 1,000 fold'reduction in themaximum dark resistance of the photoconductor coating the screen incomparison .to photoconductors employed in conventional electrostaticphotographyThe relaxation of this constraint permits the utilization ofa much wider range of photoconductor materials, particularly thosehaving higher sensitivity and/or extended red response. Evaporatedphotoconductors such as zinc cadmium sulfide, zinc cadmium selenide andcadmium sulfide, which, in the vapor deposited form, have too low aresistivity for conventional electrostatic photography are suitable forpreparing screens in the manner described in this in vention. Inaddition, the selenium alloys having extended red light response such asselenium-tellurium alloys containing more than 10 percent tellurium andselenium-arsenic alloys containing at least percent arsenic may also beemployed in this invention.

Although the asymmetrical photoconductor deposition onto either aconducting or nonconducting screen may be readily carried out by vacuumvapor deposition, it is also possible to prepare photoconductive coatedscreens using photoconductor binder layers. One preferred method offorming such a screen is to spray, using an air gun, the photoconductorbinder layer material onto the screen; the spray being directed onto thescreen at an angle. Either suitably doped and dye sensitized zinc oxideor doped cadmium sulfide dispersed in a suitable solvent with any of anumber of appropriate binders may be sprayed onto the screen or mesh atthe appropriate angle to form an effective ion control screen. Theproperties and nature of photoconductor binder layers are described indetail in the book, Xerography and Related Processes" by Dessauer andClark (pages 119-168) publishedv 1965, Focal Press Limited. Aspreviously indicated, because of the requirement for a relaxation of thedark resistivity requirement. the concentration of photoconductorpigment in the binder that may be employed on screens is significantlyhigher than that which must be utilized for the standardelectrophotographic processes. This permits the fabrication of higherphotosensitivity surfaces.

Organic photoconductors of this type are described in the previousreference (pages 169-199) may also be employed as photoconductorssuitable for the present invention.

As previously mentioned, the screen pattern on the chargeable member maybe eliminated by operating at electric fields sufficiently low that thescreen is not resolved on the chargeable member. In inexpensivecommercially available wire or plastic monofilament screens having verysmall mesh sizes or high mesh counts, the weave is found to be somewhatnonuniform, i.e., there are small random variations in mesh spacingwhich result in mesh irregularities appearing in the image developedupon the chargeable member. These small irregularities, which appear asmesh lines in the 'copy, may be eliminated by moving the screen over avery slight distance during the exposure. One manner of effecting thismotion is illustrated schematically in FIG. 5. Here, the modulatingscreen 16 and a series of corona wires 18 are mounted together on arigid framework 40. This framework is supported in such afmanner that itmay be translated transversely from left to right. The frame is alsospring loaded so that it is urged against cam 42 which is driven by lowspeed rriotor 44. Motor 44 is energized during the exposure sothatthecorona wires and screen move relative to the dielectric receptor sheetduring theexposure. Motions as small as 0.1 inch are generallysufficient to eliminate all screens nonuniformities from the developedimage. It has been found that the maximum screen velocity during anexposureis approximately 1 to 2 inches per second, depending upon theintensity of the corona current to the screen and the nature of theresponse time of the photoconductive coating. The screen may beadvantageously moved in two directions as by a circular or figure 8motion. In addition to eliminating screen variations from the developedimage, the technique of moving the screen during an exposure alsoprovides the advantage of eliminating the development of other screendefects such as random dirt and dust which settle upon the screen. Thisprocedure which essentially integrates spatially over a photoconductorduring exposure to eliminate photoconductor defects from appearing inthe developed visible image is believed to be unique.

FIG. 6 illustrates a means for moving the screen during an exposure andfor simultaneously providing for the replenishment of new screen withinthe exposure region. As shown screen 46, having a width of between 4 and18 inches, depending upon the copy size desired, and coated with anasymmetrical photoconductor is supplied from supply drum 45. A takeupdrum 437 collects the screen after it passes through the exposure area.In operation, during each exposure, the screen advances a distance ofapproximately 1/16 inch. In this way for every few hundred copies thatare made, the screen is completely replaced with previously unusedscreen from drum 45. With many vacuum coated screens several thousandsof copies have been formed from each screen with no degradation of theprocess. However it is possible that with certain very highphotosensitivity screens some fatigue may be present. The screen motion,in order to eliminate irregularities in the direction of both screenwires or monofilaments, should be such that the motion is not in thedirection of either wire or monofilament. A preferred direction of themotion is at an angle of 45 with each wire. Besides a linear motion,orbital motion or a zigzag motion may be employed to successfullyeliminate screen nonuniformities from the image and mechanism in placeof cam 42 to provide such motions is readily available.

FIG. '7 illustrates schematically an apparatus for obtaining full colorprints employing the techniques of this invention. One of the majorproblems in generating full color prints, employing color separationprinciples, is associated wth registration of the three colors. A minorproblem involves the complexity and expense in handling the copy sheet.The apparatus of FIG. 7 circumvents these difficultiesby sequentiallygenerating a charge image pattern and developing the three primarycolors without moving the charge receptor layer. In this figure, theprojector 22 serves to project a color transparency onto the coronamodulation screen 16. Three successive exposures are provided; one forblue, a second for red, and a third for green. The projected color isselected by placing a filter color wheel containing the three selectedcolor filters-50 between the projection lens and the screen. Thesequential operation of the three primary colors is carried out byindexing motor 52. The charge receptor sheet is developed in placeemploying a series of three liquid toners which are delivered in such amanner as to flow over the receptor sheet from manifold 54'. Solenoidactivated valves 5o select the appropriate yellow, cyan or magentaliquid toners which are contained in gravity fed reservoirs The liquiddeveloper,- after passing across the surface of the charge receptorsheet, is collected in reservoir 66 and discarded or else reclaimed forfurther use. v

In operation, the blue filter is indexed in front of projector 22 sothat the image corresponding to the blue tones in a color print areprojected upon modulating screen l6.'The required potentials are appliedto the corona and backing plate to form a charge image on the receptorsheet, and the solenoid valve connecting the yellow toner reservoir toapplicator manifold 54 is opened for a period of 2 to 4 seconds. Thetoner passingover the inclined surface of the charge receptor sheetdeveiops the yellow components of the image. This process is thensuccessively repeated with a red color filter using a cyan toner and thegreen coior filter employing a magenta toner. Effective liquidelectrostatic colored toners for use in this apparatus are manufacturedby the Day-Glo Corporation (Cieveland, Ohio).

An unexpected result obtained with this apparatus is the lack of arequirement for drying the charge image receptive paper betweensuccessive exposures. An image may be toned and, before the paper isdried, a second image placed on the surface.

When excess liquid remains at the surface of the charge image receivinglayer after a developer has flowed over the surface this excess liquidmay re moved by drawing a rubber squeegee or rolling a hard rubberroller over the surface of the paper. Alternately, excess liquid can beremoved from the surface with the use of an air knife.

9 Thus far, in the description of this invention, the use of dielectriccoated paper or plastic films have been indicated. In addition to thesematerials, papers fabricated from plastic (the so-called plastic papers)may also be employed in this invention. Conventional plain papersfabricated from cellulose generally contain a sufficient quantity ofmoisture and free ions so that these papers will not support anelectrostatic charge image for the time intervals required to practicethe present invention. By suitably treating plain papers, however,images maybe formed using this invention. Plain bond papers or plainblade-coated papers may be rendered sufficiently insulating by firstheating the papers to a temperature of between 120 and 200C for a periodof a few seconds. This may be carried outin a small oven. Immediatelyafter removal from the oven and while the paper is cooling down, thepaper is wet with a hydrocarbon. A preferred material for thisapplication is the aliphatic hydrocarbon solvent known under thetradename of Isopar, manufactured and marketed by the Humble Oil &Refining Company. Paper so treated is capable of sustaining anelectrostatic charge on the surface for long periods of time. A lateralsurface conductivity is still present, however, so thatonce a chargeimage has been placed on the surface of the paper, the development mustbe carried out within a period of l to 2 seconds if excessive resolutiondegradation is to be avoided.

The latentelectrostatic image formed by the corona modulation screen mayalso be employed in recording an image using a deformable thermoplasticfilm composed of polystyrene, St'aybelite, Piccolastic or otherdeformable synthetic polymer material. After the formation of anelectrostatic image on the film surface, the latent image isdeveloped bysoftening the film by known in the art.

In addition to providing a permanent image, the coexposure to eitherheat or solventl'vapor s'as is well rona modulating screens of thisinvention may also be employed with electric field sensitive cholestericliquid crystal filmsin display applications. Here; the dielectric coatedpaper of FIG. I isreplaced by a liquid crystal film and the supportplaten 10 replaced by a glass sheet having a transparent conductivecoating on the side adjacent the film. Under the influence of anelectric field provided by ions reaching the free surface of the liquidcrystalv film, the optical scattering and/or reflective properties ofsaid film are modified, leading to the formation of a visible display onthe film. Cholesteric materials suitable for this application aredescribed in British Patents, 1,123,117 and l,l67,-486, and also by L.Melamed and D. Rubin, Appli. Phys. Lett. 16, 4, 149 (1970) and by .l..l. Wysocki, J. Adams, and W. Haas, Phys. Rev. Lett. 20, I9,1,024-(1968).

FIG.-8 illustrates an apparatus in which the toned image is first formedon an intermediate endless belt and subsequently transferred to a plainpaper sheet or Web. In this drawing an endless plastic belt 62,preferably fabricated of polyseter and containing a conductive coatingonthe inside surface issupported on rollers 64 and'7,0."An electrostaticimage is formed on this belt using a corona modulation screen, coronawire and projection source in a manner similar to that shown in F IG. 1.After the electrostatic image has been formed, it is developedbyimmersion in liquid developer tank 66. A counterelectrode 68 ismaintianed at a suitable potential inorder to minimize the developmentof background. In the region between roller 64 and W, the developedimage is partially dried and then offset onto a paper sheet or web 72 asthe paper and plastic film are held in contact by rollers 70. The imageis fixed on thepaper and some residual solvent removed as the paper isheated by radiant heater 74-. Cleaning brush 76 removes residual tonerfrom the plastic endless belt.

Rather than employ the endless plastic belt (as shown in FIG. 8), aconductive drum coated with a hard insulating surface, such as aglass-based enamel, may be employed. In apparatus employing a drum, theoperational steps are the same. The electrostatic image is formed usinga corona wire and a corona modulating screen; the electrostatic image istoned, employing either a dry or liquid electrostatic developer; theimage transferred by offset to a plain paper sheet or web; and the drumcleaned. This apparatus is rather complicated but does possess severaladvantages over conventional plain paper electrostatic photography. Aprincipal advantage is the fact that the photoconductor is never inphysical contact with either a developer material or paper and, hence,is not subject to the usual wear which occurs in standardelectrophotographicplain paper copies. An insulating surface -enameleddrum possesses a hard abrasion resistant surface and hence has a lifesignificantly greater than a typical selenium drum. Devices employing anendless plastic belt would be subject to a higher degree of wear;however, the belt may be readily changedandis relatively inexpensivecompared to aselenium drum.

FIG. 9 is a schematic drawing of an apparatus for simultaneouslycharging, exposing and developing. The apparatus shown here is identicalto the apparatus in FIG. 1 with the exception that provision forinjecting an aerosol into the region between corona, modulating screen16 and a paper image receptor sheet 81 l'lffi; been added. The imagereceptor sheet in this apparatus does not require a dielectric coatingupon its surface. In order that uniform development occur, it isnecessary that the development aerosol be injected with a high degree ofuniformity into the region between screen 16 and receptor sheet 81. Theair velocity of injection cannot be too high or a displacement andbreakup of the image occurs. In addition, the aerosol must be initiallyuncharged or, if the aerosol particles are charged, the charge must beadjusted to some low value in order to minimize background. The chargepotential of the aerosol may be controlled within certain limits byadjusting the potential of the conducting manifold from which theparticles are ejected. This is accomplished with power supply 83, asshown in FIG. 9 Alternately, the particle charge may be controlled byinduction, in which case the potential of power supply 83 is notconnected directly to the conducting manifold, but is rather connectedto an electrode immediately adjacent to the aperture or slit in theaerosol generating nozzle 82.

In operation, potentials are applied to appropriate electrodes, theimage is projected onto the corona modulating screen, and the aerosol isinjected into the region between screen and receptor sheet all processesoccurring simultaneously. The aerosol particles, being essentiallyneutral, are not affected by the strong electric field and pass throughthe region defined by screen 16 and receptor sheet 81. As coronagenerated ions passthrough the screen, these ions interact with theaerosol, charging the aerosol particles which are subsequently drawnonto the receptor sheet.

While the aerosol generation embodiment shown in FIG, 9 involves the useof an air gun type atomizer 80, the invention is not restricted to thisgeneration technique. Alternate means of forming a jet involve directlyatomizing a liquid through fine jets or thermally volatizing a materialto form an aerosol cloud.

In addition to using either a liquid aerosol or a thermally volatilizeddyestuff, aerosol developmennemploying a solid powder, the so-calledpowder cloud development may be employed. Methods for generating powderclouds and details of powder cloud development are described in theDessauer and Clark reference cited earlier, pages 309 through 340. Animportant difference between the use of a powder cloud in the presentinvention and powder clouds associated with conventional electrostaticphotography involves the fact that, in the process of the presentinvention the aerosol powder cloud should be uncharged or the charge perparticle should be maintained at a rather low value.

Dyestuffs which may be successfully vaporized from a hot surface to forma uniform aerosol cloud include brilliant oil blue, oil brown 0, and oilbrown N.

FIG. 10 is a drawing of a modification of the apparatus shown in FIG. 1which enables the process to be employed with plain paper. Theelectrostatic image development, employing a liquid toner, occursessentially simultaneously with charging and exposing. A shallow metaltray 84, having rubber seals 86, contains a conventional liquidelectrostatic toner 90 which is continuouslyrecirculated through thesystem by inlet and outlet tubes 91 and 92, respectively. A plain paperweb 88 passes over the rubber seals 86. The liquid electrostatic tonerlevel is maintained so that it is in contact with the paper web. Thecorona modulating screen 16 is spaced between 84 inch and 1 inch abovethe surface of the paper. The corona source, power supplies, andillumination source are similar to those shown in FIG. 1 and are notshown here.

Radiant heater 93 is provided for heating the paper, in order to driveresidual moisture from the paper, prior to its being employed in theprocess of FIG. 10.

If conventional liquid electrostatic toners of the type employed in zincoxide paper machines are utilized in the apparatus of FIG. 10, it isfound that the paper picks up residual toner in uncharged areas, leadingto an overall grey background. This problem has been overcome bydiluting these commercially available liquid toners with carrier solventin an amount of 8 parts solvent to 1 part toner. At this dilution, thesolids content is near 0.1 percent. A majority of commercially.available liquid electrostatic toners employ aliphatic hydrocarbonsolvents as the liquid carrier. Effective dilutions may, therefore, becarried out employing the aliphatic hydrocarbon solvent lsopar G,manufactured by Humble Oil and Refining Company.

The spacing between the bottom of developing pan 84 and the lowersurface of web 88 is critical. If the spacing is too low, insufficientdensity is developed in the image, while if the spacing is too great,low density images are also observed. Optimum spacings appear to rangefrom 0.050 inch to 0.300 inch. Optimum results are obtained underconditions such that the exposure time is short, generally one second orless. These conditions are realized by employing an illuminationintensity at the corona screen of 5 ft.-candles or greater and highcorona current which is obtained by running the corona wires at highpotentials and spacing the wires reasonably close to control screen 16.

in certain highly absorbent papers, a background image is observed evenat low toner dilutions. This is caused by the takeup of developerparticles into the surface of the paper as the toner is absorbed by thepaper. This background may be eliminated with the addition of auxiliaryroller 87 which supplies a pure aliphatic hydrocarbon solvent to the webprior to the web contacting the liquid developer. The solvent, typicallylsopar G, is fed to roller 87 as this roller revolves through a pan 8%containing the solvent. Since theweb is already saturated or prewet withpure solvent, no developer takeup occurs in the paper, thus resulting incleaner backgrounds.

It has been found that, to a first approximation, the density of a tonedimage is roughly proportional to the charge per unit area that isdeveloped. A charge density of approximately 0.15 [.LCOUL/Cfll isrequired to develop a dense image. The potential to which a chargesupporting member must be charged in order to develop this chargedensity is inversely proportional to the capacity of the per unit areaof the chargeable member. Dielectric papers, having a dielectric coatingthickness of approximately 6 microns, develop dense images when chargedto potentials of 300 volts, corresponding to a charge density close to0.15 pcouL/crn? If the charge is developed across a 3 mil sheet ofpaper, the surface must be charged to potentials in the region of 3,000to 4,000 volts to obtain this charge density. In view of thisrequirement, it has been found necessary to employ high potentialsbetween screen 16 and developer container 84. Minimum potentials of 15ltv are required with 20 kv resulting in higher resolution images havingless distortion. It has further been-found that 13 rate of developmentis proportional to the surface voltage. Thus, for the apparatus of FIG.10, even though the toner has been substantially diluted over thatnormally employed in electrostaticxerographic processes,

the development time is extremely short; generally less than 1 second,because of the high surface potentials developed due to the low capacityper unit area of paper compared to the thin dielectric coating utilizedwith dielectric coated paper.

FIG. 12 illustrates yet another process for employing a photoconductivecoated screen together with devel opment apparatus to generate a visibleimage errlploying plain paper. In this drawing, screen 16, powersupplies, illumination source, etc., are similar to FIG. 1. A paper web98 is supported by paper drive rollers (not shown) so as to be spaced avery slight distance above conducting roller 94. This roller serves in amanner similar to paper backing plate 10 of PEG. 5, and is electricallyconnected to power supply 23. Rotter 94 revolves, the lower surfacepassing into tray 5 containing an ink 96 dispersed or dissolved in apolar liquid. During operation, roller 9d revolves carrying up a thinfilm of ink over its surface. The roller and paper web speeds areadjusted so that the web surface speed is equal to the velocity of theperiphery of roller Since this is a dynamic process, means are providedfor moving the image from left to right across screen It: at the samevelocity as the paper moves from left to right. Thus there is speedcorrelation between the image projected on screen 16, paper web 98, andthe motion of the ink film on roller 94 directly below the paper. Assurface charge develops on the upper surface of paper web 98, an intenseelectrostatic field is developed between the paper and conducting roller94. The high electrostatic forces generated in the gap between thebottom of the paper and the ink film cause the ink to jump from theroller to the surface of the paper, thereby forming a permanent visibleimage. A,.wide variety of inks are effective in this process, includingalcohol and waterbase inks consisting of colloidal carbon dispersions,opaque dye pigments, or dissolved acid or basic dyesfFor effectiveoperation, the gap spacing between the ink film and the lower surface ofthe paper must be maintained uniform and, for typical operatingconditions, between the extremes of 2 mils and 50 mils. An operating gapof mils appears preferable. With certain inks and at certain velocities,it is difficult to establish a uniform ink film thickness on the surfaceof roller 94. In this event, thickness control attachments well known tothe art, such as doctor blades or reverse rolls, may be added toestablish the proper ink film thickness. This process has the advantage,in addition to using ordinary paper, of generating an extremely clean,background since no ink or developer touches the paper in areas whichare not charged. For certain papers, under high hunidity environmentalconditions, the paper web must again be preheated so that the chargeplaced upon the surface of the paper does not diffuse within a period ofa few tenths of a second. This process functions effectively withstandard weight plain papers since very high potentials, on the order of2,000 to 3,000 volts, are placed on the surface paper and the spacingbetween the top surface of the paper and conducting roller 94 is only afew thousandths of an inch. These high potentials established acrosssuch a short distance result in high electrostatic'forces beingdeveloped at the surface of the ink film; such forces being sufficientto locally draw the ink film across the prints s p- It has been foundthat, when the surface of a charge receptor member is charged tovoltages high in comparison to the voltage existing between screen 16and backing plate I0, image distortion occurs. This distortion arisesfrom fields at the surface of the chargeable member; these fields,existing between a charged and uncharged region. This results in coronagenerated ion beam bending in a manner so as to reduce the width ofuncharged lines. Secondly, when a high potential is built up in asignificantly large area, a reduction in the local field immediatelybelow screen 16 occurs in this region with subsequent diffusion of ionspassing through the screen in this region. This effect is not seriousfor low charging voltages, particularly when high potentials are appliedbetween screen 16 and backing electrode 10. In charging relatively thickplastic films, which requires high surface voltage potentials (severalthousand volts, for example, in case of 3 to 5 mil polyester or acetatefilm) these distortins are observed.

A means forcircumventing this problem is shown in FIG. 12. Thisapparatus is identical to that shown in FIG. I but includes a secondfine mesh screen 100 whose potential. is established by power supply102. This fine mesh conducting screen is spaced very close to thesurface of'the chargeable member, generally within a distance of 5 to 25mils. The screen-potential, as established by power supply 102, ismaintained between the potential of backing plate and screen l6.

beam diffusion and distortions mentioned previously.

We have found that, because of the high system resolution, moirepatterns are formed in the image corresponding to screen mesh overlapbetween screen 100 and screen 16. In order to eliminate this problem,

. screen 100 may be vibrated or caused to move by employing a motor andcam assembly 104 operating in a manner similar to that shown in FIG. 5.

FIG. 13 illustrates a further means of employing a corona currentmodulating screen to realize simultaneous charging, exposing, anddeveloping while using a plain paper. Endless conducting belt 108,supported and driven by conducting rollers 110, is employed to support alayer of developer or toner particles and is positioned, with uniformspacing, immediately below paper web 88. The potential of the conductingrollers and the conducting endless belt is established by power supply21. A uniform thin film of dry toner particles is continuously suppliedto the endless belt from hopper 112 containing a reservoir of tonerparticles 114. After traversing the development area, toner particlesfalling off of the endless belt are collected, for reuse, in tray 116.The operation of the apparatus shown in this figure is similar to thatof FIG. ll. The'rnotion of the endless belt andpaper may be continuous;in which case the image to be reproduced must be scanned across screen16 to match velocityof paper web 88, or the machine 1 may operatein astepand repeat" mode; the paperand .endless belt advancing betweensuccessive exposures.

' Eithera conducting or nonconducting toner may be employed with thisapparatus; the toner being transferred from the endless belt to theunderside of the paper webby virtue of the high electrostatic forcesexisting at charged regions of the paper. The same precautions regardingthe endless belt paper spacing indicated for apparatus of FIG. 12 arepertinent for this apparatus.

FIG. 14 schematically illustrates an apparatus employing a coronamodulation screen 16 in such a manner as to form a visible toned imageon a plain paper sheet which is supported on backing plate 10. The imageprojection source, corona wire, screen, and backing plate are identicalto the apparatus as shown in FIG. 1, and are connected to power suppliesas shown in FIG. I. In this device, charging and toning of the paperimage are carried out simultaneously by employing an open mesh screen122 moving through -toner reservoir 130. The open mesh screen 122consists of a fine mesh (generally I00 to 300 meshes per inch) formed asan endless belt and traveling over drive pulley 124, idler pulley 126,and pulley 128 which carries the open mesh-through a toner supply 132.In opera-' tion, the open mesh web is driven through toner to provide atoner laden mesh surface immediately adjacent the paper upon which theimage is to be developed. During an exposure, ions passing through theion corona current modulating screen impinge upon the open mesh screencarrying toner, charging the toner particles to a high potential, andthe toner particles are subsequently electrostatically attracted ontothe plain paper sheet 120. Toner is thus deposited on the paper in areascorresponding to regions in which corona current traverses modulatingscreen 16. Either liquid or dry toners may be employed in thisapparatus, although best success has been realized with the use of drytoncrs. 'l'he toners are not. in general, electrostatically held ontothe mesh screen 122 but are collected mechanicallyv It has been foundadvantageous, in instances wherelow toner pickup on the open mesh end--less belt 122 is observed, to very slowly move the belt during theexposure. This provides an additional toner source as toner is depletedfrom the belt. Optimum belt drive speeds are-in the range of l-/32.to reinch per secnd Although the apparatus shown in FIGS. 7, 8, 9, l0, l2,l3, and 14 are described as being employed with corona modulated screensdiscussed in this invention, thesedevelopment means will also functionwith multilayer screens which are first charged and then exposed andthen employed to modulate a subsequent corona discharge as described inUS. Pat.v No. 3,582,206. When employed in this manner, an advantage ofdevice simplicity is obtained over that described in the abovereferenced patent. v

The following examples illustrate the techniques of the method,'processand apparatus described in this disclosure. These examples are not meantto be restrictive in any way however.

EXAMPLE 1 A plain square weiiwiofimsh" phosphor bronze screen wasstretched over a square brass frame whose inside dimension was 4incheson a side and whose out side dimension was 5 inches. The phosphor bronzescreen was soft soldered onto the frame. The frame was mounted in avacuum coater a distance 12 inches from a quartz crucible mounted intantalum heater. The screen was inclined 45 from the normal. A charge of30 grams of xerogrophic grade. selenium was placed in the evaporationcrucible. The system was evacuated to a pressure of torr andlhe seleniume ap ra te d from the boat onto the screen over a periodof 45 minutes.During the evaporation, the screen was heated, with an electricalheater, to a temperature of 70C. The selenium coating thickness wasfound to be microns.

The screen was removed from the vacuum evapora- The contrastratio,defined here as the ratio between the ion current to theconductivebacking plate 10 with the photoconductive screen in the darkand ioncurrent with the same screen illuminated, was determined by"connectinga Keithley Model 6m A electr ometer betweenpapersupporting-electrode l0 and power supply 21. At a counterelectrodepotential of 3. kv anda corona potential of +l0 kv, the dark current was8.3

pamperes and the'current obtained whenthe screen was uniformlyilluminated with tungsten illumination at a level of IO ft. -candles was1.5 #amperes. The contrast ratio was thus 5.5. At a corona potential of+8 kv,

the dark current was 3.7 iamperes and the light current was 0.45ampercs; yielding a contrast ratio of 5 8.2.

It may be seen from the aforementioned measure ments that a highercontrast potential is obtained at lower corona potentials. In thisevent, however, the col0 rona current is lower, and longer exposuretimes are required to charge the dielectric paper. At a screen-paperseparation of inch, an applied potential of -3kv is sufficient toaccelerate the ions to the surface of a dielectric coated paper andstill maintain a resolution of l 3 line-pairs/mm in the developed image.

Copies of a projected image were obtained by placing sheets ofdielectric coated paper on the paper counterelectrode' 10. An imagehaving a high-light brightness of 10 ft.-candles was projected on thescreen with a simultaneous application of corona and counterelectrodepotentials; the total exposure time being 3 seconds. The paper was thenremoved from the counterelectrode, immersed in a beaker of liquid tonerhaving a solids concentration of 1 percent in lsopar G. Since the papersurface was charged positively and since the liquid developer tonerparticles are also positively charged, a reversal imagewas obtained.After the paper was removed from the developer, excess liquid wassqueegeed from the surface and the paper dried in an airstream which mayor may not be heated. The

image was of high quality, having negligible background and a maximumdensity ofv 1.1. The develop ment time was 3 seconds. I

A selenium coated ion current modulating screen wasprepared in a manneridentical to that of Example I l with the exception that the evaporafiodtilasTafiied out with the screen mounted normal to the line ofevaporation. When evaluated in the apparatus of FIG. 2, it was foundthat, at a corona wire potential of +l0 kv, the dark current was 7.3,uamperes and the current with an illumination level of i0 ft.-candleswas 4.6 pamperes; providing a contrast ratio of only 1.6. A number ofattempts were made to obtain satisfactory copies of the light image in amanner described in Example 'lffln no'case was it possible to obtain ahigh contrast between light and dark areas on the paper.

;High background Ievelswere obtained together with low image density.

Example 2 illustrates results obtained employing the .teachings of US.Pat. No. 3,220,324. This example is t 5 0 included to indicate theadvantages realized when practices the teachings of the presentinvention.

The following examples illustrate the diversity of photoconductormaterials, deposition methods, and geometry of screen arrangementssuitable for utilization in the present invention.

TABLE I Examples -Photnconductor Selenium Selenium Selenium SeleniumSelenium-Tellurium D I Selenium-Arsenic* Deposition Vacuum Vacuum VacuumVacuum Vacuum Vacuum Evaporation Method Evaporation EvaporationEvaporation Evaporation Evaporation Photocondu'ctor 20 2O 20 20 20Thickness (u) Angle of Deposit 45 90 60 30 45 45 (from normal)() Screen325 mesh dacron 325 mesh dacron 325 mesh dacron 325 mesh dacron 325 meshnylon 325 mesh nylon Conductor Layer Al evap. 90 to Al evap. 90 Al evap.90 Alevap. 90 Al evap 90 normal Al evap. 45 normal normal oppositenormal opposite normal opposite normal opposite opposite oppositephotoconductor photoconductor photoconductor photoconductorphotoconductor photoconductoi' Light Level 2 5 2 2 2 2 (ft-candles)Contrast Ratio 9.2 6.7 8.0 7.6 8.8 9.1

Evaporated alloy consisting of l2 atomic percent telluriurn and 88atomic I a Q h prior to "Evapor'ated alloy consisting of 50 atomicpercent selenium and 50 atomic percent arsenic formed by n L prior to rPhotoconductor Cadmium sulfide Cadmium selcnide Zinc oxide-plioliteSelenium Zinc oxide-pliolite Selenium binder binder Deposition VacuumVacuum Air gun spray Vacuum Dip coat and air Vacuum evaporation Methodevaporation evaporation evaporation blast to clear openingsPhotoconductor l0 I0 20 25 20 Thickness (11.)

Angle of Deposit 45 5 45 90 45"; screen rotated (from normal )t) duringevaporation s r 325 mesh nylon 325 mesh nylon 325 mesh stainless 325mesh dacron 250 mesh 325 mesh stainless steel Conductor Layer Al evap.45 to Al evap. 45to None Al evap. 90 to None None nonnal opposite normalopposite normal on both photoconductor photoconductor sidesto completely7 7 cover filament T Light Level 2 2 l0 2O 20 :(fL-Candles) ContrastRatio"- 8.1 6.4 NJ 6.0 6.2 L6

5 l6 'corona assembly was mounted in a cross slide and spri- 4O rpho'oconducmcadmium Sulfide selenium Selenium rig loaded in such amanner that the assembly was free Polycarbonate to travel back andforth. A small. gear head motor operbinder ating at 60 rpm was employedto drive a cam having an eccentricity of /4 inch. When the motor wasenergized, Method screen immersed evaportion evaporation in dispersionthe screen described a lateral motion having an ampliv rz tude of itinch. The corona wires were operated at a :2? potential of +l4kv, theconducting layer on the screen Photoconduc- I0 20 was grounded, and thepaper support platen was operzza lilgcknm ated at a potential of l 5 kv.A microfiche image, hav- Angle f 45 45 45 ing clear letters on adarkened background, was imaged Denali! (from on the screen at amagnification ratio of approximately Screen ,325 mesh 7 2 mil copperParallel wire grid e lg lg m e micro "P stainless steel sh eethaving 2of 5 mil wires on was 4 ft. -candles. Exposures were made by placrng asheet of dielectric coated paper upon the conducting Conductor None NoneNone metal platen and simultaneously projecting the image t l lft Levels I 20 2o 55 on the screen and applying potential to both the coronamama) wires and paper support platen. The total exposure Contrast Ratio]l- 7.6 8.]

EX M LE l8 The screendescn bed i'nExample 3 was mounted in a frame as'shown in FIG. 5. A Plexiglas arch mounted on opposite sides of the screensupported 35 mil diameter corona wires. The corona wire to screenspacing was 34 inch. The screen was supported t inch above a 6 inchsquare brass paper support platen. The screen time was second. After theexposure, the paper was then removed from the support platen anddeveloped by immersion in a liquid toner consisting of a fine dispersionof carbon particles in lsopar G hydrocarbon fluid. A'small quantityofdissolved resin present in the developer provided a fixing action whenthe paper was heated to volatize the 'lsopar solvent. A variety ofdielectric papers were evaluated in this apparatus and found to functioneffectively; high density images with clear backgrounds being obtainedwhen the images were developed in a liquid toner having positivelycharged carbon particles. Such dielectric papers, available from anumber of manufacturer'sycons'ist of a paperb ase rendered conductingthrough the incorporation of certain additives and over which is coateda thin plastic film generally having a thickness of from O.l to mil.

In all cases the images were crisp and sharp; a resolution of 6 to 10lines-pairs/mm being routinely obtained.

Images were also developed employing transparent plastic. films.Polyester films of l to S'mil thickness were successfully employed aswere acetate films in the same thickness range. For plastic films whichdo not possess a conductive. backing, an auxiliary backing platemust beemployed. Such a plate was constructed of 15 mil thick aluminum. Thepolyester film was placed upon the conductive backing plate and theassembly placed together upon the brass support platen.

An exposure was made and the polyester film and conducting backingassembly removed together from the exposure apparatus and immersed inthe liquidtoner together to develop the latent electrostatic image.

The utility of moving the screen corona assern bly during l'exposure wasestablished with this apparatus.

No screen patterns due to screenweaving irregularities wereobserved-when the screen was moved during'an exposure. In addition,defects due to dusta nd'dirt on the screen were eliminated from thefinal copy. If the screen was not moved during exposure,'a series ofvery 40 faint lines running in both directions corresponding to thescreen filaments were observed'in the developed image.

Excellent continuous tone images were obtained by 5 projecting acontinuous tone transparency onto the screen. Good-solid area coverageafter development was observed. It is thought that the solid areacoverage is obtained since the photoconductive coated screens serves afunction of screening the image. A very fine screen pattern is typicallyobserved in large dark developed areas. 1 I

;;;.z..: Q... z

i The apparatus of the previous examplewas modified by tilting the wholeapparatus at an angle of 30? to the horizontal undudding a developermanifold, solenoid operated valves and developer supply tanks as shownin FIG. 7. In addition, provisionswere made for placing color filters infront of theslide projector 22. The developer manifold consistedof a /4inch diameter copper tube (6 inches long) sealed at one end. Holes 0.020inches in diameter were drilled in a line along the distributionmanifold at a spacing of /4- inch. The open end of the copperdistribution manifold was connected to electrically operated solenoidvalves 56 withsuitable unions. Each of the three solenoid valves wereconnected in turn to'a liquid developer reservoir. A collection pan 60was provided to collect the liquid developer after it had flowed overthe surface-of the paper being developed. I

"Afull color positive transparency was placed in projector 22 and ayellow filter placed in front of the projector. The high-light intensityat the screen was 20 ft. -candles'. The first exposure was made in 1second, employing a negative corona potential, and immediately afterexposing, thesolenoid valve connecting the developer distributionmanifold to the tank containing the yellow liquid. toner was opened fora period of 2 seconds. During" this period, the yellow toner flowedacross the dielectric paper which had previously been placed on thebrass paper support platen. Immediately after the yellow developmentoperation was completed, the red filter was placed in front of the slideprojector. The light intensity was increased until a high-lightintensity of 60 ft. -candles was incident upon the screen and the imageexposed with the potentials applied to screen and backing electrode fora period of 1 second. The solenoid valve connecting the developermanifold to the cyan liquid toner reservoir was opened again for aperiod of 2 seconds. The process was continued a third time employing agreen filter with a high-light intensity of 20 ft. -candles and a 1second exposure followed by a 2 second magenta development. The paperwas then removed from the conducting brass platen and dried in a warmair stream. 'A positive print was thus obtained havinga good colorbalance and a high degree of registration. I

. In several experiments it was found that the charged colors hadrunsomewhat. This problem waseliminated by removing excess liquiddeveloper from the surface of the paper aftereachdevelopment operation.It was found that this liquid removal could be effectively carried outby employing either a rubber squeegee, a /4 inch diameter, rubber rollerwhich was run over the dielectric paper, or by directing a high velocityjet of air over the surface of the paper to blow excess developer fromthe paper surface.

"EXAMPLE 20 The schematic drawing of FIG. 8 illustrates a means foremploying this invention with any plain paper. The apparatus, as shownschematically in FIG. 8, was constructed employing a 6 inch wide Mylarendless belt 30 inches in circumference. The 5 mil thick polyester filmwas rendered conducting on the inner surface by vacuumdepositing a layerof aluminum thereon. Drive puileys 64 were constructed of solidbrass 3inches in diameter. A spring loaded idler pulley (Zinche's in di- 55ameter) loaded the endless belt to maintain tautness.

T l1e p l otoc c nd uc tor coated screen, corona wire, and projectorsetup describe in Exariiplei tiwasemployed and mounted in a verticaldirection as shown in FIG. 8; the screen to endless belt separationbeing is inch. A conducting development immersion tray 66 waselectrically connected to drive pulleys 64 which, irrturn,ter'ssasieasrs'saiehssrar T5 kv. outin exposure, the corona wireswereconnected toa potential of +15 kv. A negative image was projected on thescreen and the tray 66 was filled with a liquid developer containingpositively charged carbon colloidal particles. The toner image formed bysequentially exposing a portion of the Mylar ball belt to the modulatedcorona image followed by liquid development was offset onto a plainpaper web 72 with the aid of pressure roller. 70. The image was fixed onthe surface of the paper by heating with radiant heater 74. This heaterconsisted of a General Electric Quartzline infrared 'lamp mounted in apolished aluminum reflector. The residual image remaining on the endlesspolyester belt was cleaned with the soft fur brush 76, revolving at aspeed of 500 rpm in contact with the surface of the belt. in operation,the drive motor is turned off andan image projected upon thephotoconductor screen while the proper high voltage is applied to form alatent electrostatic charge image onthe surface of the polyester belt.The drive motors are then energized, providing an and less belt speed of3 inches per second. The electrostatic latent image is developed as itpasses through developer bath and the image is offset onto the paper bymeans of pressure roller 70. Image transfer may be assisted by operatingpressure backing rollers 70 behind the paper at a potential positivewith respect to the pressure roller behind the polyester web. After theimage has been transferred to the paper, the polyester belt drive ishalted and a second exposuremay be generated andthe process repeated.

' Under certain operating conditions it is found that a slight residualcharge imageis present on the polyester surface. This residual image waseliminated by employing a polonium radioactiveantistatic device. Theradioactive source was positioned a fraction of an inch above the weband extended across theweb'inthe region between cleaning brush,76 andthe photoconductor coated screen. I j j i I n EXAMPLE 21 I Apparatus asshown in FIG. 9 was assembled. The backing plate-photoconductor coatedscreen-corona projection source employed in Example 18 was used in thisexperimental arrangement with the screen ind backing plate mounted in avertical direction. The apparatus of FIG. 1 was modified to include oneadditional power supply and a means forgenerating anaerosol which wouldtraverse across the face of a receptor sheet during anexposure,.thus'providing for simultaneously charging, exposure, anddevelopment. By this means it is possible to employ ordinary plain paperas a receptor sheet for charged aerosol particles. No further processingis required otherthan the heating of the receptor sheet to fix the tonedimage. A number of approaches were employed for generating a neutrallycharged aerosol suitable for employment in this example.

One technique, involved the use of a Nichrome wire vmils in diameter.This wire was precoated with a film of DuPont Oil Brown 0, a dark-brownanthraquinone dye. The wire was positioned midway between the 556-toconductive screen and the receptor sheet near the bottom of. thescreen. During the exposure and while potentials were applied to thecorona wires and the paper conductive backing current was'passed throughthe nichromewire, raising its temperature just below red heat. The.anthraquinone dye rapidly volatized without decomposition from the'wireand, because of the thermal currents generated, traversed-the spacebetween the screenand the plain paper receptor sheet. The potential ofthe wire, neglecting the small potential drop required to'heatthe wire,was at ground. it was found that positive ions traversing the screen inunexposed areas deposited a charge upon the aerosol. resulting in theformation of a visible print as the oil dye deposited upon the receptorsheet due to the electrostatic forces present.

In another experiment, an aerosol of dry powder was generated employinga distributor manifold having small apertures and extending across thebottom of the opening defined by the photoconductive coating screen andthe plain paper backing electrode. A number of both wet and dry aerosolswere employed in this apparatus, pressurized gas driving the aerosolthrough the distributor manifold. Carbon blacks, colored pigmentparticles, and both conducting and nonconducting inks were employed. Thepotential of the pigment particle system, as shown in FIG. 9, iscontrolled by power supply 83. Depending upon the triboelectric chargingproperties, this potential was adjusted to minimize background density.

in additional experiments, the power supply outputwas not connecteddirectly to the conductive manifold (which was maintained at groundpotential) but was connected to a bar electrode mounted immediatelyabove and to the side of the aerosol ejection manifold.Conductiveaerosolsyejected through the distribution manifold ports, werethus charged by induction; the potential of the particles depending uponthe power supply 83 potential.

EXAMPLE}? Th apparatus of FIG. 10. was'assembled. The corona modulatingscreen of Example 3 v was employed to modulate the corona current to acontinuous receptor web 88. The corona wire assembly imea'ris formovingthe screen, etc.,. were identical to that employed in previous examples.The liquid toner reservoir M was 5 inches square and contained inlet andoutlet tubes ill and 92 through which a 1 percent solids content,conventional, electrostatic tonerwas circulated. A large O-ring seal 86was cemented to the top of toner reservoir 84 to confine theliquid tonerto the region immediately below web 88.

Excellent results were obtained in a step and repeat mode using a 3 milpolyester film as web 88. In the case of the polyester film, preheatingand presolvent wetting was not required. Exposure times, at a screenillumination intensity of 10 ft.- candles, ranged from we to 2 seconds.I 7

T we of x le fiw mgs ifisqw the addition of a continuous belt cornfisedof a fine mesh screen which was mechanically supported so as to bedriven across the surface of the paper as shown in FIG. 14. The screensevaluated were formed into a continuous belt 5 inches wide a'nd20 incheslong. The screen was driven by a metal cylinder drive roller 12$, 2inches in diameter. ldler rollers 126 and 128 positioned the screen andprovided support for the screen as the screen was driven through toner,l32, contained in toner reservoir 130. Samples were prepared under avariety. of conditions ranging from the case in which a screen wasstationary in front of the paper during an exposure to situations inwhich the screen was driven at speeds to 2 inches per second past thesurface of the paper during "the exposure. Both metal, phosphor bronze,and stainless steel as well as dacron and nylon screens wereevaluated;the screen mesh sizes ranging from 200 mesh to 325 .mesh. It was foundthat both the nonconducting and conducting scre'ens wereequallysatisfactory in fon'ningimages corresponding to the image projected ontothe corona modulating screen on the plain paper sheet 120. y

In operation, the corona modulating "screen was maintained at'groundpotential and the plain paper'support platen'was maintained at kv. Anumber ofexperime'nts were carried out in which thetonercontai'ning-screen to paper spacing was varied, and no substantialdifference in image quality was found over spacings from a few mils toas inchi Both conventional liquidelectrostatic toners and dry carbonpowders were employed sa'tisfactorily in this apparatus. A number offinely divided carbon particles were evaluated and excellent resultswere'obtained by Van Dyke Corporation gold seal toner for 3,600 Copier.The light intensitiesv and exposure timeslrequired were, in most cases,similar to that of Example 18 V. i I

In the case of the metal screen, good results were obtainedwhen thescreenwas operated at potentials in the region of 5 to 8 kv. Equallygood results were obtained when the s'creen-roller-toner reservoirassembly was left floating, i.e., not connected directly to anypotential. In this case, the screen assembly probably was stabilized toa potential near -1 0 kv due to capacitive coupling between the screenand paper support platen 10. I v Y We claim: I I I I ,I

1. In an apparatus for preparing electrostatic images conforming to anoptical image on a'ch'argea'ble, image receptor surface, which includes:7

an electrically conductive platen adapted to support an image receivingmember;

a corona source adapted tospray ions on saidimage receiving member; andI a corona modulating screen disposed adjacent said image receiving"member between "said corona source and said image receiving member; theimprovement which comprises providing as said corona modulating screen,a screen consisting of a plurality of strands and a photoconductivecoating on at least a portion of the outer surfaces of said strands forspatially modulating the flow of ion current from said corona source tosaid chargeable image receiving member, said coating being asymmetricalwith respect to said strands by being offset from a plane passingthrough said strand, and perpendicular to said screen, when saidscreenisviewed in cross sectionff I I I 2. The apparatus of claim 1- including,in addition, means to maintain said electrically conductive platen at aselected potential; a power supply electrically connected to said coronasource and means for simultaneously providing-electricity tosaidplatemsaidscreen and said corona source concurrently withprojection-of a light image onto said image receiving member.

3. The apparatus of claim including means to project and to focus said,image on said image receiving member. I I I I 4. The apparatus ofclaim 1. including means to move said corona source in a plane parallelto said ion modu-f lating screen during projection of said'imageontosaid image receiving member.

5. The apparatus of claim 1 including means to move both said coronasource and said ion modulating screen in planes parallel to said imagereceiving member during projection of said image onto said imagereceptor surface.

6. The apparatus of claim 1 including means'to sup ply fresh arraywithin an exposure region in order to provide for continued andrepetitive operation of said apparatus. i

' 7. The apparatus of claim 1 including, in addition, means to develop avisible image on said image receptor surface. I I

8. The apparatus of claim 7 wherein said means to develop a visibleimage'is a means to develop a colored image. 1

9. Theapparatus of claim 1 including, in addition, means for heatingsaid charge receiving member and for wetting said charge receivingmember while it retains someof said heat prior to projection of an imagethereon. v v I I 7 10. The apparatus of claim 7 including, in addition,means to transfer the visible image developed on said charge receptorsurface onto a permanent record member. I 11'. The apparatus of claim 1including in addition, a second screen positioned very closely adjacentto the surface of said image receptor member, and means to maintain saidsecond screen at a suitable potential.

12. The apparatus of claim 1 wherein said charge receiving member isaconductive drum coated with an insulating layer. I I

13. The apparatus of claim 1 wherein said ion modulating screen is anendless belt.

14.- The apparatus of Claim 1 wherein said ion per-, meable membe'r'isan apertured plate.

ln a n appa r atu s for preparing visible images on I receptor sheetconforming to an optical image projected onto an ion permeable memberhaving a photosensitive coating thereon which includeszanion source; tan imagereceptor sheet'disposed so as to receive ions from said source;I anion permeable member disposed between said source and-said receptorsheet and being coated at least in part with a photosensitive material,said coating being asymmetrical with respect to said member -by beingoffset from a plane passing through'said member and perpendicular to theplane of said member, whensaid member is viewed incro'ss'sectiom I v IIII means to project an optical imageonto said ion permeablernernber; I

means for inaintaining desired electrical potentials between said.receptor sheet, said ion source and said ion permeabie member duringprojection of said p al'ima e; an

.m eans to develop a visible imag e on said image renter h t a 1 6 Theapparatus of claim 15 whereinisaid means to develop ajyisible imageincludes means. for projecting an aerosol into the region between saidionrnodulating member and said image receptor vmember. simulta neouslywith the operation of the remainderofsaid apparatus, W I

-17. The apparatus of claim wherein the means for, projecting an aerosolis a means to project a fine cloud 25 26 of solid particles havingsubstantially no electrostatic tive surface being on side of imagereceptor sheet opcharge thereon. posite the ion permeable array.

18. The apparatus of claim 16 wherein the means to project an aerosolprojects a fine cloud of liquid particles having substantially noelectrical charge thereon.

20. The apparatus of claim 19 wherein the conductive surface is on aroller at least partly immersed in a liquid ink bath.

19. The apparatus of claim including means to The apParatuS of chum 'P fmage support an image receptor Shaet in close proximity to ceptor sheetis contacted, on the side of said sheet opbut not in physical contactwith a conductive surface Posite the Permeable member, with? liquidelectrocoated with a thin film of liquid ink, the ink having a 10 Staliceveloper.

conductivity less then 10 ohm-cm and saidconduc- UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No. 3,797,926 Dated March 19,197

Inventor(s) Richard A. Fotland and Virgil E. Straughan It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Columns 17 and 18 in Table in Example 4, sixth line down "90" shouldread--80-- In Examples 3, l, 5, 6 and 7 in Table I, the conductor layershould read--Al evap. perpendicular to the plane of the screen onto theside opposite the photoconductor--instead of "Al evap. 90 to normalopposite photoconductor" In Example 12 the conductor layer shouldread--Al evap. perpendicular to the plane of the screen on both sides tocompletely cover filament--1nstead of "A1 evap. 90 to normal on bothsides to completely cover filament" Signed and sealed this 6th day ofMay 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officerand Trademarks 'RM PO-IOSO (10-69) USCOMM'DC O37Q P69 U.S. GOVIINIIINI'PRINTING OFFICI "I! O-lOl-lll,

Patent No. 3,797,926 Dated March 19, 197 4 Inventor-(s) Richard A.Fotland and Virgil E. Straughan It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Columns 17 and 18 in Table I, in Example L Sixth line down "90" shouldread--80....

In Examples 3, '4, 5, 6 and 7 in Table I, the conductor layer shouldread--Al evap. perpendicular to the plane of the screen onto the sideopposite the photoconductor--instead of "Al evap. 90 to normal oppositephotoconductor" In Example 12 the conductor layer should read--Al evap.perpendicular to the plane of the screen on both sides to completelycover filament--instead of "Al evap. 90 to normal on both sides tocompletely cover filament" Signed and sealed this 6th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officerand Trademarks FORM PO-1050 (10-69) uscoMM-oc 60376-P69 ".5. GOVERNMENTPRINTING OFFICE I90! O-SiG-IM,

1. In an apparatus for preparing electrostatic images conforming to anoptical image on a chargeable, image receptor surface, which includes:an electrically conductive platen adapted to support an image receivingmember; a corona source adapted to spray ions on said image receivingmember; and a corona modulating screen disposed adjacent said imagereceiving member, between said corona source and said image receivingmember; the improvement which comprises providing as said coronamodulating screen, a screen consisting of a plurality of strands and aphotoconductive coating on at least a portion of the outer surfaces ofsaid strands for spatially modulating the flow of ion current from saidcorona source to said chargeable image receiving member, said coatingbeing asymmetrical with respect to said strands by being offset from aplane passing through said strand, and perpendicular to said screen,when said screen is viewed in cross section.
 2. The apparatus of claim 1including, in addition, means to maintain said electrically conductiveplaten at a selected potential; a power supply electrically connected tosaid corona source and means for simultaneously providing electricity tosaid platen, said screen and said corona source concurrently withprojection of a light image onto said image receiving member.
 3. Theapparatus of claim 2 including means to project and to focus said imageon said image receiving member.
 4. The apparatus of claim 1 includingmeans to move said corona source in a plane parallel to said ionmodulating screen during projection of said image onto said imagereceiving member.
 5. The apparatus of claim 1 including means to moveboth said corona source and said ion modulating screen in planesparallel to said image receiving member during projection of said imageonto said image receptor surface.
 6. The apparatus of claim 1 includingmeans to supply fresh array within an exposure region in order toprovide for continued and repetitive operation of said apparatus.
 7. Theapparatus of claim 1 including, in addition, means to develop a visibleimage on said image receptor surface.
 8. The apparatus of claim 7wherein said means to develop a visible image is a means to develop acolored image.
 9. The apparatus of claim 1 including, in addition, meansfor heating said charge receiving member and for wetting said chargereceiving member while it retains some of said heat prior to projectionof an image thereon.
 10. The apparatus of claim 7 including, inaddition, means to transfer the visible image developed on said chargereceptor surface onto a permanent record member.
 11. The apparatus ofclaim 1 including, in addition, a second screen positioned very closelyadjacent to the surface of said image receptor member, and means tomaintain said second screen at a suitable potential.
 12. The apparatusof claim 1 wherein said charge receiving member is a conductive drumcoated with an insulating layer.
 13. The apparatus of claim 1 whereinsaid ion modulating screen is an endless belt.
 14. The apparatus ofClaim 1 wherein said ion permeable member is an apertured plate.
 15. Inan apparatus for prepariNg visible images on a receptor sheet conformingto an optical image projected onto an ion permeable member having aphotosensitive coating thereon which includes: an ion source; an imagereceptor sheet disposed so as to receive ions from said source; an ionpermeable member disposed between said source and said receptor sheetand being coated at least in part with a photosensitive material, saidcoating being asymmetrical with respect to said member by being offsetfrom a plane passing through said member and perpendicular to the planeof said member, when said member is viewed in cross section; means toproject an optical image onto said ion permeable member; means formaintaining desired electrical potentials between said receptor sheet,said ion source and said ion permeable member during projection of saidoptical image; and means to develop a visible image on said imagereceptor sheet.
 16. The apparatus of claim 15 wherein said means todevelop a visible image includes means for projecting an aerosol intothe region between said ion modulating member and said image receptormember simultaneously with the operation of the remainder of saidapparatus.
 17. The apparatus of claim 16 wherein the means forprojecting an aerosol is a means to project a fine cloud of solidparticles having substantially no electrostatic charge thereon.
 18. Theapparatus of claim 16 wherein the means to project an aerosol projects afine cloud of liquid particles having substantially no electrical chargethereon.
 19. The apparatus of claim 15 including means to support animage receptor sheet in close proximity to but not in physical contactwith a conductive surface coated with a thin film of liquid ink, the inkhaving a conductivity less then 1012 ohm-cm and said conductive surfacebeing on side of image receptor sheet opposite the ion permeable array.20. The apparatus of claim 19 wherein the conductive surface is on aroller at least partly immersed in a liquid ink bath.
 21. The apparatusof claim 15 wherein the image receptor sheet is contacted, on the sideof said sheet opposite the ion permeable member, with a liquidelectrostatic developer.